I assume that, at a certain point, residency on Mars would be functionally permanent. For the same reason we're not having a conversation about colonizing Jupiter, we wouldn't be speculating about sending people back from Mars to Earth.

Past that - and assuming you could solve the umpteen zillion other problems associated with permanent residency on a foreign planet - this would be one of the most radical experiments in speciation since domestication began. I have to assume that the first generation on Mars would have an atrocious mortality rate. But subsequent generations would adapt, just as they have adapted on Earth for the last two billion years.